Aircraft engines for modern military and commercial aircraft require large quantities of air at very high-flow velocities and, as a result, are prone to the creation of an inlet vortex when operating at high power settings. Most often, this happens when the aircraft is operating at low speeds or stopped on the ground.
The suction created by turbofan or turbojet engines causes the formation of a stagnation point on the ground due to the asymmetric flow lines at the bottom of the engine inlet. An inlet vortex is formed when low velocity horizontal flow, such as wind, near the ground plane adds an additional velocity vector that is perpendicular to the normal stagnation flow lines. The horizontal flow near the ground is superimposed onto the vertical upflow of air from the ground and a vortex will immediately form, creating a swirling effect. The vortex that is created beneath the engine inlet is analogous to the vortex caused by the forces of nature, and increased intensity of either velocity vector will increase the magnitude and power of the vortex. The position in relation to the aircraft at which the vortex meets the ground plane is not a fixed point, but varies with external wind conditions.
When this vortex is formed there is a considerable reduction in pressure at the center of the vortex, thereby causing the vortex to act like a concentrated tornado, "vacuuming up" debris, dirt, stones and ice, creating Foreign Object Debris (FOD). When ingested into a turbofan or turbojet engine, FOD can damage engine components and cause mechanical failure of the engine by colliding with internal engine parts such as the engine impeller or fan blades. The closer to the ground that the aircraft engine hangs from the aircraft wing, the greater the intensity of the vortex and the more problematic FOD becomes.
An early solution to the FOD problem utilized a large "vacuum cleaner" to sweep the runways of airports where planes would operate, clearing the path of the airplane of all debris and potential FOD. cleansweeping runways, however, is not fullproof, requires expensive equipment, many man hours of time and is useless on gravel or unpaved runways. Accordingly, a better approach to eliminating FOD is through aircraft engine design. However, design of an engine that minimizes the potential for engine failure due to ingestion of FOD is a substantial challenge.
Engine designs address the inlet vortex problem in different ways while attempting to maintain engine thrust and efficiency. In particular, some conventional designs attempt to solve the inlet vortex problem by either reducing the intensity of the vortex or destroying the inlet vortex completely.
As an illustration, U.S. Pat. No. 3,298,637 for an "Engine Inlet Protective Screen Arrangement" discloses a conventional jet engine modified at the intake to include an encircling enlarged hollow conduit containing a series of holes around the circumference of the conduit. Pressurized air is discharged through the holes to create an air screen to restrain or block any dust or water which would tend to be thrown up by the engine during take-off. This "protective screen arrangement" is a continuous flow system that creates a wall of air to prevent the stagnation point from forming, requiring a large amount of bleed air flow from the engine. Although the '637 patent appears to solve the stagnation point problem, creation of the wall of air will also cause a large amount of debris to become airborne, potentially causing more FOD problems, and sacrifices thrust needed to back an aircraft. The configuration also requires a series of channels and doors to duct and direct the air flow, increasing the complexity and cost of the system.
Likewise, U.S. Pat. No.3,474,988 discloses a "Pod For A Gas Turbine Engine" to reduce ingestion of debris into the engine inlet. The "pod" is a front fan pod and has a part-cylindrical panel which swings down and forward from the pod. The purpose of the panel is to present a debris-free surface to the air intake of the engine so that the vortex is formed on the panel, and not on the ground in front of the engine intake. The '988 patent contemplates that a complex mechanical system be deployed into the inlet flow of the engine. At a minimum, deploying a mechanical system into the inlet flow makes the system vulnerable to FOD, and potentially, creates FOD due to part breakage and ingestion into the engine.
Also noteworthy is U.S. Pat. No. 3,527,430 which discloses "Protective Air Curtains For Aircraft Engine Inlets" to prevent ingestion of stones and debris into the aircraft engine when the aircraft is on the ground. The air curtain is blown from a forwardly-projecting tube to disrupt the inlet vortex and isolate the intake air flow from the region of the ground where the stagnation point forms and from where loose material might be lifted. The system disclosed in the '430 patent requires continuous flow which demands a large amount of the core compressor bleed air flow from the engine to be directed down to the runway surface, potentially kicking up additional debris which can be ingested into the engine as FOD. Moreover, the positioning of the device in front of the engine inlet will increase the drag of the aircraft and cause potential disturbances in flight to the aircraft wings and flaps.
U.S. Pat. No. 3,905,566 describes a "Jet Engine Intake Protection System" for forming a fluid barrier in proximity to the lower leading portion of the forward intake end of an aircraft engine. The fluid barrier is projected forward of the intake end of the engine and is parallel to the direction of flow of air into the engine, thereby attempting to prevent ingestion of matter into the engine intake. This approach seemingly recognizes the deficiencies of the previous patents by providing a relatively simple design with few, if any, moving parts, and creates a wall of air via continuous flow through many slots in the leading edge of the engine. However, this approach uses continuous brute flow force to create a barrier, draining the engine of power, increasing engine temperatures as power settings increase and disadvantageously impacting engine life.
Notwithstanding the numerous prior systems for reducing FOD ingestion by an aircraft engine, there is still a need for a simple, low cost, highly reliable aircraft engine inlet vortex disruption system that disrupts the engine inlet vortex and eliminates the creation of FOD at low or static aircraft speed while maintaining engine power and efficiency.